The present invention relates to a per-channel codec for a PCM (pulse-code modulation) transmission system.
A codec is utilized for translating (encoding) an analogue voice signal into an appropriate PCM code at a channel's transmission end, and for restoring (decoding) the analogue voice signal from the transmitted digital data at the receiving end. In these a/d and d/a conversions, the codec requires operation clocks (coding and decoding clocks) which are produced more than a predetermined number of times (for example, more than twelve times) per period of sampling pulses (frame pulses).
In a conventional codec, since the operation clocks are formed by a fixed frequency divider to which read/write clocks for a input/output register of the codec are applied, the frequency of the operation clocks is fixed at a certain value which is determined by the frequency of the read/write clocks and also by the dividing ratio of the fixed frequency divider. Therefore, the conventional codec cannot be commonly adopted in various transmission systems which use read/write clocks having respective frequencies which are different from one another.
In order to solve the above-mentioned problem, there is known a codec which is provided with plural fixed frequency dividers having dividing ratios which are different from each other and a selection circuit, and which obtains the operation clocks by selectively applying the read/write clocks to the appropriate fixed frequency divider. However, this codec requires many fixed frequency dividers and requires an additional signal for selecting the divider. Furthermore, in order to make the best use of a sampling period, during the a/d and d/a conversion, it is necessary that all of the dividers of this codec be designed in different circuit constructions. As a result, not only does the circuit design become difficult, but also, the hardware of the codec becomes complex.